1. Field of the Invention
The present invention relates to a tilt detector for detecting a tilt which occurs between an information recording surface of a storage medium and an optical axis of a light beam used for recording and/or reproduction, when information is optically recorded on the storage medium and/or information is optically reproduced from the storage medium.
2. Description of Related Art
Recently, there is broadly known a technique in which a light beam such as a laser beam and the like is applied to a disc-shaped storage medium to optically record information onto the storage medium and/or to optically reproduce information already recorded on the storage medium therefrom. At the time of recording and/or reproducing information on and/or from a disc-shaped storage medium using the light beam, the angle between the information recording surface of the storage medium and the optical axis of the light beam may sometimes shift from the right angle (i.e., 90 degrees) to induce a tilt in a radial direction of the storage medium (this tilt occurring in the radial direction of the disc-shaped storage medium will be hereinafter referred to as xe2x80x9cradial tiltxe2x80x9d). For example, a centrifugal force in the storage medium revolution and a deflection of the storage medium itself due to an aged change may be the causes of the radial tilt. If information is recorded or reproduced in the presence of the radial tilt, aberration (mainly coma-aberration) takes place within the light beam irradiated range on the information recording surface of the storage medium. This disables the precise control of light-spot size, and hence high-density information recording becomes difficult. Therefore, it is necessary to detect the quantity and the direction of the radial tilt and compensate for it. For example, in a most general radial tilt detection method employing a tilt sensor, a dedicated light beam for radial tilt detection is irradiated on the information recording surface, separately from the light beam used for information recording and/or reproduction. The light beam reflected by the surface is received by a light detector including multiple light detecting portions divided by a divisional line arranged in parallel with the circumferential direction of the storage medium, and the quantity and the direction of the radial tilt are obtained from the difference of the received light quantities of the respective light detecting portions. Namely, if the difference is equal to zero, there is occurring no radial tilt. If the difference is not zero, there is occurring a radial tilt in the light beam, which has the direction corresponding to the polarity of the difference and the quantity corresponding to the absolute value of the difference.
There is known a disc-shaped information storage medium so-called DVD-RAM (DVD-Random Access Memory) which was standardized recently and has an ability to record and/or reproduce information repeatedly for many times. The DVD-RAM is an improvement of DVD which has much larger recording capacity than CD and enables repetitive recording and reproduction for multiple times. In an apparatus for recording and/or reproducing information on and/or from DVD-RAM, no radial tilt detection and compensation function has been employed. This is mainly because DVD-RAM has the same size as CD, and large radial tilt which needs its compensation rarely took place. However, in order to further improve the accuracy of information recording/reproduction for the DVD-RAM, it is preferred to compensate for the radial tilt even if it is small.
Supposing that the conventional tilt sensor (i.e., the above-mentioned dedicated tilt sensor which additionally irradiates dedicated light beams for tilt detection) is used to detect the radial tilt taking place in relation to the DVD-RAM, the tilt sensor needs high-accuracy detection function due to the fact that the tilt itself is small, thereby increasing the cost of the information recording and/or reproducing apparatus. Further, additionally providing the dedicated tilt sensor increases the adjustment steps of the tilt sensor itself in the manufacturing process of the information recording and/or reproducing apparatus, thereby declining the productivity. Still further, the detection accuracy of the radial tilt may decline due to aging of the tilt sensor.
It is an object of the present invention to provide a tilt detector capable of accurately detecting the radial tilt without providing a dedicated tilt sensor.
According to one aspect of the present invention, there is provided a tilt detector adapted to be used for a storage medium provided with a recording track on which information is recorded and a first and a second header portions, each arranged in a manner shifted in opposite directions to each other from a center line of the recording track, the detector including: a light irradiation unit for irradiating a light beam onto the first header portion, the second header portion and the recording track; a light receiving unit having a first light receiving surface and a second light receiving surface arranged adjacently to each other on both sides of a divisional line which is in parallel with the direction of the center line and for receiving the light beam reflected by the storage medium, the first light receiving surface outputting a first output and the second light receiving surface outputting a second output; an operation unit for executing an arithmetic operation of the first output and the second output to generate an operation result signal; and an error signal generation unit for generating a tilt error signal eased on the operation result signal, the tilt error signal indicating a tilt between the storage medium and an optical axis of the light beam.
The above tilt detector receives the light beam reflected by the first header portion and the second header portion formed on the storage medium with the shifts in opposite directions to each other from the center line of the recording track, and detects the tilt between the storage medium and the optical axis of the light beam using the reflected light. Therefore, the tilt can be detected without providing a dedicated tilt sensor.
The error signal generation unit may include a low-pass filter which extracts a low-frequency component of the operation result signal as the tilt error signal. By this, the quantity and the direction of the tilt can be accurately detected with a simple configuration.
The error signal generation unit may include an averaging circuit which extracts a D.C. component of the operation result signal as the tilt error signal. Thus, the quantity and the direction of the tilt can be accurately detected.
In a preferred embodiment, the storage medium may include a disc-shaped storage medium, the recording track may include a pre-groove portion and a land portion, the first header portion may be arranged in a manner shifted by a half track pitch in a first radial direction of the disc-shaped recording medium from a center line of the pre-groove portion, and the second header portion may be arranged in a manner shifted by the half track pitch in a second radial direction, opposite to the first radial direction, of the disc-shaped recording medium from the center line of the pre-groove portion. With this arrangement, the tilt in the radial direction of the disc-shaped storage medium can be accurately detected without the use of a dedicated tilt sensor.
According to another aspect of the present invention, there is provided a tilt detector adapted to be used for a disc-shaped storage medium provided with a recording track on which information is recorded and header areas on which predetermined address information is recorded, the recording track including a pre-groove portion and a land portion, each of the header areas including a first header portion arranged in a manner shifted by a half track pitch in a first radial direction of the storage medium from a center line of the pre-groove portion and a second header portion arranged in a manner shifted by a half track pitch in a second radial direction, opposite to the first radial direction, of the storage medium from the center line of the pre-groove portion, the tilt detector including: an irradiation unit for irradiating a light beam onto the header areas and the recording track; a light receiving unit having a first light receiving surface and a second light receiving surface arranged adjacently to each other on both sides of a divisional line which is in parallel with the direction of the center line and for receiving the light beam reflected by the storage medium, the first light receiving surface outputting a first output and the second light receiving surface outputting a second output; a reproduction signal generation unit for generating a first reproduction signal corresponding to the address information recorded in the first header portion and a second reproduction signal corresponding to the address information recorded in the second header portion based on the first output and the second output; a delay unit for delaying the first reproduction signal by a predetermined time period to generate a delay signal; an operation unit for executing an arithmetic operation of the delay signal and the second reproduction signal to generate an operation result signal; and an error signal generation unit for generating a tilt error signal based on the operation result signal, the tilt error signal indicating a tilt between the storage medium and an optical axis of the light beam.
The above tilt detector receives the light beam reflected by the first header portion and the second header portion formed on the disc-shaped storage medium with the shifts in opposite directions to each other from the center line of the recording track, and detects the tilt between the storage medium and the optical axis of the light beam using the reflected light. Therefore, the tile can be detected without providing a dedicated tilt sensor.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.